I can swim but I can't fly. (I'm able to swim but I'm not able to fly.)
Must.- 100% sure, no doubt about it.
Might.- 50% sure. You are hesitating.
Can't.- 0%. No way. That's impossible
1.- Have to / Must
Similar but a bit Different
If you hesitate or doubt, use have to. It is always correct!
2.- Don't have to / Mustn't
Be careful, here meanings are totally different:
3.- Must / Should
1.- Personal Opinion.- Are you tired? Maybe you should go home and take a rest.
2.- Objective Opinion.- Ann, look at your face. You ought to go home and take a rest.
1.- May I go in?
2.- May I erase the blackboard?